1. Field of the Invention
The present invention relates to a zoom lens system having an image blur compensation function, and more specifically to a zoom lens system having an image blur compensation function for use in a camera for shooting silver-halide film pictures, a camera for shooting video pictures (moving and still pictures), or a single-lens reflex camera, for example.
2. Description of the Prior Art
Conventionally, various types of zoom lens systems having an image blur compensation function have been proposed. For example, Japanese Laid-open Patent Application No. H7-27978 proposes a zoom lens system that is composed of, from the object side, a positive lens unit, a negative lens unit, a negative lens unit, and a positive lens unit and in which a single lens element included in the fourth lens unit or a doublet lens element constituting the third lens unit is moved in a direction perpendicular to the optical axis so as to compensate for an image blur. Moreover, Japanese Laid-open Patent Application No. H5-224160 proposes a zoom lens system that is composed of, from the object side, a positive lens unit, a negative lens unit, a positive lens unit, a positive lens unit, and a negative lens unit and in which the fifth lens unit is divided into a front lens unit having a negative optical power and a rear lens unit having a positive optical power. Here, image blur compensation is achieved by moving the front lens unit (composed of a plurality of lens elements). Furthermore, Japanese Laid-open Patent Application No. H7-318865 proposes a zoom lens system that is composed of, from the object side, a positive lens unit, a negative lens unit, a positive lens unit, a positive lens unit, and a negative lens unit and in which a doublet lens element constituting the fourth lens unit is moved in a direction perpendicular to the optical axis so as to compensate for an image blur. In addition, Japanese Laid-open Patent Application No. H8-114771, by the same inventors as the present invention, proposes a zoom lens system that is composed of, from the object side, a positive lens unit, a negative lens unit, a positive lens unit, and a positive lens unit and in which image blur compensation is achieved by moving a single lens element constituting the fourth lens unit.
In the zoom lens systems proposed in Japanese Laid-open Patent Applications Nos. H7-27978 and H7-318865 mentioned above, an aperture stop is provided in the fourth lens unit. On the other hand, in the zoom lens systems proposed in Japanese Laid-open Patent Applications Nos. H5-224160 and H8-114771 mentioned above, an aperture stop is provided in the third lens unit. In some of such conventional zoom lens systems, an image blur compensating lens unit and an aperture stop are provided in one zoom unit, and this inconveniently causes interference between the driving mechanism for driving the aperture stop and the driving mechanism for image blur compensation. To avoid this, it is inevitable to make the entire zoom unit including the driving mechanisms unduly large. This spoils compactness of the entire zoom lens system, making the camera as a whole unduly large. Similarly, in a zoom lens system in which a focusing lens unit and an aperture stop are provided in one zoom unit, or in a zoom lens system in which a focusing lens unit and an image blur compensating lens unit are provided in one zoom unit, interference between the driving mechanism for focusing and the driving mechanism for driving the aperture stop or for image blur compensation cannot be avoided unless the camera as a whole is made unduly large.
Moreover, in the zoom lens system proposed in Japanese Laid-open Patent Application No. H5-224160 and the like, the type of aberration called the off-axial image-point movement error, which is one of the aberrations caused by camera shake, is not properly corrected. Even if satisfactory imaging performance is secured during image blur compensation, large off-axial image-point movement errors occur unless distortion is properly corrected. This causes blurring of an image in its off-axial area when image blur compensation lasts for a relatively long time. In addition, since the front lens unit that is moved to achieve image blur compensation is composed of a plurality of lens elements, even though it is possible to correct the axial lateral chromatic aberration that occurs during image blur compensation, the weight of the image blur compensating lens unit is too large to be comfortably borne by the driving mechanism for image blur compensation. In contrast, in a zoom lens system that has an image blur compensating lens unit composed of a single lens element, the weight to be borne by the driving mechanism for image blur compensation is minimal, but it is impossible to correct the axial lateral chromatic aberration that occurs when decentering is effected. Furthermore, with some zoom lens systems for which no specific focusing method is disclosed, it is difficult to secure satisfactory imaging performance for a closest distance object because they do not provide appropriate focusing solutions despite having an image blur compensation function.